The invention relates to a device for recording or displaying pictures, comprising a cathode-ray tube having, in an evacuated envelope, a target and a semiconductor cathode having a semiconductor body with a major surface on which a first electrically insulating layer having at least an aperture is provided. The semiconductor body comprises at least a pn-junction in which by applying a voltage in the reverse direction across the pn-junction electrons can be generated in the semiconductor body by avalanche multiplication which emanate from the semiconductor body at the area of the aperture in the first electrically insulating layer and in which at least an accelerating electrode is present on the first insulating layer at least at the area of the edge of the aperture in said layer.
Such a device is known from Netherlands patent application No. 7905470 laid open to public inspection on Jan. 15, 1981.
The invention relates in addition to a device for recording or displaying pictures, comprising a cathode-ray tube having in an evacuated envelope a target and a semiconductor cathode having a semiconductor body with at a major surface a p-type surface zone provided with at least two connections of which at least one is an injecting connection at a distance from the major surface which is at most equal to the diffusion recombination length of electrons in the p-type surface zone.
Such a device is known in Dutch Pat. No. 150,609 published on Aug. 16, 1976 (corresponding to British Pat. No. 1,147,883).
In addition, the invention relates to a semiconductor device for use in such a device.
In a device for recording pictures, the cathode-ray tube is a camera tube and the target is a photosensitive layer, for example a photoconductive layer. In a device for recording pictures, the cathode-ray tube may be a display tube, while the target comprises a layer or a pattern of lines or spots of fluorescent material. Such a device may also be designed for electronlithographic or electronmicroscopic uses.
In Netherlands Patent publication No. 7905470, a cathode-ray tube is shown having a so-called "cold cathode". The operation of this cathode is based on the emanation of electrons from a semiconductor body in which a pn-junction is operated in the reverse direction in such manner that avalanche multiplication of charge carriers occurs. Some electrons may obtain so much kinetic energy as is necessary to surpass the electron work function; these electrons are then released at the major surface of the semiconductor body and thus provide an electron current.
Emanation of electrons is facilitated in the device shown by providing the cathode with so-called accelerating electrodes on an insulation layer present at the major surface which do not cover a (slot-shaped, annular, circular, rectangular) aperture in the insulating layer. In order to further facilitate the emanation of electrons the semiconductor surface is provided, as desired, with an electron work function-reducing material, for example caesium.
Because residual gases always remain in the evacuated envelope, negative and positive ions are liberated from said residual gases by the electron current. The negative ions are accelerated in the direction of the target. In the case of electrostatic deflection they may be incident on a small area of the target and damage same or disturb its operation. In order to prevent this detrimental effect, ion traps are used. An ion trap for negative ions is known, for example, from U.S. Pat. No. 2,913,612.
Under the influence of accelerating and focusing fields prevailing in the tube, a part of the positive ions move in the direction of the cathode. When no special measures are taken, a part thereof will be incident on the semiconductor and damage the same in that a kind of ion-etching takes place.
This damage may involve a gradual etching away of the electron work function-reducing material. By a redistribution or even total disappearance of this material the emission properties of the cathode vary. When said layer is not present (or is removed entirely by the abovementioned etching mechanism) even the major surface of the semiconductor body may be attacked. In a semiconductor cathode based on avalanche multiplication of charge carriers as described in Netherlands patent application No. 7905470 in which the emitting p-n junction extends parallel to the major surface and is separated therefrom by a thin n-type surface zone, it is possible that as a result of said gradual etching the said surface zone disappears entirely so that the cathode no longer functions. In a similar type of cold cathode, as described in Netherlands patent application No. 7800987 laid open to public inspection on July 31, 1979, the p-n junction is exposed at the major surface of the semiconductor body. As a result of the above-described damaging action of positive ions present in the electron tube, for example, the place where the p-n junction is exposed at the major surface may vary. This causes an unstable emission behavior.
In the second type of cathode-ray tube in which in the semiconductor cathode a p-n junction is operated in the forward direction, the so-called negative electron affinity cathode (NEA-cathode), the emission behavior is also influenced in that ion etching again takes place. In this case also, first the layer of electron work function-reducing material is gradually etched away. The p-type surface zone of the cathode is then attacked until the cathode no longer functions.
It has been found that the above-mentioned processes can occur so rapidly that the life of cathode-ray tubes manufactured with such semiconductor cathodes is considerably shortened hereby.